In high data rate optical transmission systems, the maximum transmission distance is determined by variation in the optical frequency of the pulses output by the modulator.
In an optical transmission system using as its emitter an integrated component having a laser section and a modulator section, referred to below as an integrated laser modulator or ILM, such frequency variation has, in fact, three components: transient frequency variation, also known as transient "chirp"; adiabatic frequency variation, also known as adiabatic chirp; and oscillation, also known as oscillating chirp.
An ILM device is shown diagrammatically in FIG. 1. It is a semiconductor device having at least one laser oscillator section 1 and at least one modular section 2. The section 1 includes a distributed grating 3 known as a Bragg grating, extending along the longitudinal propagation axis and delivering to the modulator section 2 light energy of power P.sub.l at a given frequency (or at a given wavelength .lambda..sub.l) selected by the distributed grating, in response to electrical current I1 being injected transversely through the laser section.
The modulator section 2 is an electro-absorbent section controlled by a modulator voltage V1 representative of a signal to be emitted. This section is treated to have a forbidden bandwidth greater than that of the laser section so that the laser wavelength .lambda..sub.l is several nanometers longer (e.g. 10 nm to 50 nm) than the wavelength .lambda..sub.m of the modulator section. Thus, the modulator section when biased by a zero level for the voltage V1 (V1=0) is transparent to the light emitted by the laser section, while it becomes completely absorbent for a voltage level that is negative. This on/off modulation is performed by the voltage signal V1 which corresponds to the stream of bits to be transmitted at the transmission frequency of the optical transmission system. Thus, a "1" bit is transmitted by applying a voltage level 0, while a "0" bit is transmitted by applying a negative voltage level. Encoded light pulses are thus obtained at the outlet face of the modulator section. The outlet face 4 of the modulator is given antireflection treatment so as to have a low reflection coefficient R2.
As mentioned above, that ILM device suffers from three types of light frequency variation.
Transient frequency variation is frequency variation on the high and low transitions of the pulses due to on/off modulation. This on/off modulation gives rise to variation in the refractive index in this section at the times of the high and low transitions. This gives rise to phase variation at the outlet face of the modulator. This is what varies the pulse frequency on the rising and falling fronts of the light pulse, accompanied in practice by a red offset or a blue offset at the outlet from the modulator section.
Adiabatic frequency variations and oscillation are due to the modulator and the laser being in an integrated structure. Although the outlet face 4 of the modulator is given antireflection treatment, it still has a reflection coefficient R2 that is not zero. Light loss in the modulator is therefore modulated. The residual modulation of this light loss gives rise to variation in carrier density in the laser section. This causes the refractive index in the laser section to vary, and so the laser wavelength .lambda..sub.l changes. The static component of this variation is the adiabatic frequency variation which consists in different frequencies at the stabilized high and low levels of a light pulse output from the modulator. The dynamic component of this variation consists in oscillations on each of the high and low levels.
These frequency variations are shown in FIG. 2. They correspond to a stream of output light pulses from the modulator section. It is possible to distinguish transition frequency variation on the rising fronts .DELTA.f.sub.th and on the falling fronts .DELTA.f.sub.tb. Adiabatic frequency variation, which in the example shown has a positive amplitude .DELTA.f.sub.a of about one gigahertz, and oscillations .DELTA.f.sub.osc on the high and low states can also be seen. Unfortunately, these frequency variations have a direct affect on optical transmission quality. It is therefore appropriate to control frequency variations so as to optimize transmission quality, thereby making it possible to increase the distance between repeaters on optical links.
One known solution in the state of the art for controlling transient frequency variation consists in providing a third or "phase" section after the modulator section. The phase section is controlled by a voltage signal that is in phase opposition to the control signal V1 applied to the modulator section. In this way, the refractive index variation in the modulator section is compensated, thereby making it possible to eliminate or attenuate transient frequency variation. However, that solution takes no account of problems associated with reflection on the outlet face from the modulator circuit and therefore does nothing to solve problems of adiabatic frequency variation and of oscillation on the high and low levels of the light pulses. In addition, the phase section needs to be of a composition that is different from that of the other sections (wavelength different from the laser wavelength .lambda..sub.l and from the modulator wavelength .lambda..sub.m), which means that additional manufacturing steps (epitaxy) are required. Furthermore, compensation solutions based on modulation by means of signals in phase opposition are always complex. Finally, modulation in the phase section gives rise to additional light losses.
Concerning adiabatic frequency variation and oscillation, it has been observed that oscillation amplitude decreases with decreasing amplitude of adiabatic frequency variation. Reducing adiabatic frequency variation therefore also reduces oscillation. Particular attention has thus been paid to adiabatic frequency variation. Adiabatic frequency variation varies with the value of the phase .OMEGA. of the wave on the outlet face of the modulator section. The value of the phase is itself a function of the position of the grating relative to the outlet face. Thus, the value of adiabatic frequency variation in an ILM device is itself highly random. Nevertheless, for ILM devices obtained in the same manufacturing run, a maximum amplitude for adiabatic frequency variation is known (in absolute value).
A commonly used method consists in sorting components and in retaining only those components that exhibit low adiabatic frequency variation.
To reduce adiabatic frequency variation, the outlet face of the modulator is also treated so that it is coated in antireflection materials, so as to obtain reflectivity R2 that is as small as possible. At the cost of using a complex manufacturing method, it is thus possible to achieve reflectivity R2 of about 2.times.10.sup.-4 as compared with reflectivity of about 10.sup.-3 or 10.sup.-2 as obtained by common manufacturing methods. However, combining the methods of sorting components by performance and of drastically reducing the reflection coefficient R2 has non-negligible repercussions on manufacturing costs, and in addition greatly reduces yield.
Thus, the state of the art does not provide satisfactory solutions for reducing adiabatic frequency variation and oscillation on output light pulses.
An object of the invention is to provide a simple and effective solution to this technical problem.
Experientially, it has been found that the amplitude of adiabatic frequency variation is a periodic function of the phase .OMEGA. of the wave on the outlet face of the modulator. There also exists a value for the phase .OMEGA. at which the amplitude of adiabatic frequency variation is zero (or equal to a given value). In the invention, a technical solution has thus been sought making it possible to determine the phase value .OMEGA. at which adiabatic frequency variation has the desired value and enabling action to be taken on the ILM device so as to obtain said phase value.
It is observed that the light power P.sub.l and the voltage of the laser section are modulated by the controlling voltage modulation V1 applied to the modulator section. Numerical simulation of the modulation of the power .delta.P.sub.l /.delta.V1 by modulation of the voltage V1 shows that when the phase .OMEGA. is adjusted to have zero amplitude for the adiabatic frequency variation, then modulation of the laser power P.sub.l (rear face) is at a minimum. The same applies to modulation of the laser voltage V.sub.l. FIGS. 3 and 4 are curves for .delta.P.sub.l /.delta.V1. The first curve shows strong resonance for adiabatic frequency variation having positive amplitude of about 1 GHz. The second curve shows greatly reduced resonance for adiabatic frequency variation of zero amplitude.
The technical solution to the problem posed in the invention is based on this observation. By measuring the amplitude of the voltage modulation at the terminals of the laser section, it is possible to determine and adjust the phase at the outlet from the modulator section. Thus, if the amplitude of this voltage modulation is at a minimum, for example, then the corresponding amplitude in adiabatic frequency variation will be zero.
If it is desired to set the amplitude of adiabatic frequency variation at a determined value that is not zero, it is also necessary to calibrate variation in the high frequency voltage with phase variation. Given the maximum amplitude of adiabatic frequency variation for a given manufacturing run, it is possible to calibrate the laser voltage modulation curve and thus determine the phase value that needs to be obtained.